How do I use the alpha channel of a texture for arbitrary data in Cycles?
In order to do what you are you describing, you need to make a couple changes to your node setup.


Change the Mix Shader to an Add Shader.
Unplug the texture color from the multiply node and plug the multiply node into the Strength instead of the Color of the Emission shader.  You can now use the multiply node to adjust the strength of the emissive parts of the material.